Validate CSV output schemas

Also moved code dir to src.
There are reasons to do standard things in standard ways. While it is
possible to get the `code/` directory to work, and recognize it as a
package path, this requires wrangling the pyproject.toml file.
Additionally, any import from the `code.something` path automatically
shadows the python stdlib `code` module. While it may not be necessary,
it still is good to not shadow standard library modules.

src/tests/test_validate_date_col.py

@@ -0,0 +1,10 @@
+import dataframely as dy
+import polars as pl
+
+
+class DateSchema(dy.Schema):
+    date: dy.Date = dy.Date(nullable=False)
+
+    @dy.rule()
+    def minimum_starting_date() -> pl.Expr:
+        return pl.col("date") > pl.date(2018, 5, 8)